Lift-off process for terminal metals

ABSTRACT

A process is described for selective removal of unwanted metallization from the surface of a semiconductor device. The process comprises the usual deposition of a configurable image defining layer on the surface of the device upon which a suitable pad limiting metallurgy (PLM) has already been deposited. The layer is then opened over the pad limiting metallurgy using standard techniques and coated with a layer of the terminal metal. The coated device is then heated to just above the melting point of the terminal metal causing the melted metal, through surface tension to form a ball of metal on the PLM and to form small globules of metal on the surface of the layer and then permitted to cool. When cooled the layer is removed using the usual techniques. Because the coating of terminal metal is no longer a continuous layer on the surface of the mask, removal of the polymer mask can be accomplished in about one-tenth of the time required when compared to a deposited terminal metal layer that is not melted. Also because of the effects of surface tension the metal coating need only be one-half the thickness required under the prior art techniques.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the fabrication of semiconductor integratedcircuit structures and in particular to the fabrication of the terminalmetallurgy required on the surface of such semiconductor structures.

2. Description of the Prior Art

In the semiconductor art terminal metallurgy is usually produced usingthe so-called lift-off processes, wherein a uniform layer of theterminal metal is laid down over the entire surface of the chip which iscoated by a soluble polymer in those places the metal is not to contactthe surface of the chip so that by the dissolution of the polymer themetal in the unwanted areas is lifted off the surface of the chip.

Typical lift-off processes are shown in a number of prior art patents,such as for example, U.S. Pat. Nos. 4,532,002; 4,108,717 and 4,045,594.In these processes metal deposited on the surface of a semiconductordevice is lifted off by a dissolving of the underlying organic material.

An improvement to the basic lift-off concept was described in U.S. Pat.No. 4,519,872 which sets forth a lift-off process in which theunderlying polymer is thermally depolymerized such that its dissolutioncan be more quickly accomplished since depolymerized polymer is moreeasily removed in a solvent.

Another improved lift-off process is described in U.S. Pat. No.4,428,796 in which the polymer is heated to break the bond betweendeposited metal and the polymer.

Still another improved lift-off process is in U.S. Pat. No. 4,448,636which describes a process in which the underlying polymer is heated withradiant energy, such as from a laser, to cause the polymer, under themetal film, to outgass thereby breaking the mechanical bond between themetal film and the resist.

SUMMARY OF THE INVENTION

The present invention provides an improved method for the formation andfabrication of terminal metallurgy of integrated circuits. The presentinvention can utilize various image defining layers, such as polymers orphotoresist compositions which do not polymerize significantly at highertemperatures. The present invention also permits reduction in thethickness of the deposited terminal metal layers while still maintainingthe thickness of the final metallurgy on the chip. Still further, theprocess of the present invention permits the removal of the unrequiredexcess deposited metal from the surface of the underlying insulatingimage defining layer without removing of the image defining layer, thusproviding greater physical protection as well as alpha barrierprotection to the underlying wafer and still achieve the necessaryexterior, external terminal electrical connections required to properlyconnect and drive the circuit.

A process is described for selective removal of unwanted metallizationfrom the surface of a semiconductor device. The process comprises theusual deposition of a configurable image defining layer or mask on thesurface of the device upon which a suitable metallurgy, such as padlimiting metallurgy (PLM) has already been deposited. The image defininglayer is then opened over the metallurgy using standard techniques andcoated with a layer of the terminal metal. The coated device is thenheated to just above the melting point of the layer of terminal metal onthe metallurgy causing the melted metal, through surface tension to forma ball of terminal metal on the metallurgy and to form small globules ofmetal on the surface of the image defining layer or mask and thenpermitted to cool. When cooled the image defining layer or mask isremoved using the usual procedures. Because the coating of terminalmetal is no longer a continuous layer on the surface of the mask,removal of polymer masks can be accomplished in about one-tenth of thetime required when compared to a deposited terminal metal layer that isnot melted. Also because of the effects of surface tension the terminalmetal coating need only be one-half the thickness required under theprior art techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 7 show, in section, one sequence of a process embodyingthe present invention, and

FIGS. 8 through 13 show a sequence of a different but preferred processembodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is particularly used for forming the terminalmetallurgy, i.e. contacts and metallic wiring, on the surface of asemiconductor device and will be described in reference to formation ofthe final solder contacts found on semiconductor devices.

FIG. 1, shows in section, a portion of a typical semiconductor wafer 20in which there is provided a number of semiconductor circuits 23 andwhich has completed all the processing prior to final metallization.This FIG. 1 shows the wafer 20 provided with a suitable insulating layersuch as a polyimide layer 21, which, in turn, is provided through knowntechniques such as photolithographic and etching techniques with asuitable via opening 22 leading to the underlying semiconductor circuit23. It should be noted that such circuits usually require a multiplicityof such openings in which contacts can be made but, for the sake ofillustration only, only the formation of one such contact will be shown.It should also be noted that many insulating materials can be used forthis layer and many techniques such as a laser beam cutting of the likecould be used to make the required openings therein. Following theformation of this opening 22, a layer 24 of so-called pad limitingmetallurgy (PLM) usually a micron or so thick, such aschrome-copper-gold or chrome-copper-chrome, or titanium, ortitanium-copper as shown in FIG. 2, is deposited, for example byevaporation or sputtering, over the polyimide and through the opening 22to contact the underlying semiconductor circuit 23. After this PLM isdeposited, as shown in FIG. 3, a suitable image defining layer 25 whichmay be, for example a photoresist sold commercially under the name ofDupont 2560 or Dupont Riston is applied in a thickness of between 38-100microns (1.5 to 4 mils). The use of such photoresist for such a purposeis well known to the semiconductor industry. It should be noted thatother materials can be used for this purpose such as polyimide, glass,etc. The photoresist is exposed and developed using normalphotolithographic procedures, as is well known to those skilled in theart, to create a window 26 overlying the PLM coated via opening 22provided in the polyimide layer 21. This window 26 is carefullydimensioned to define the contact area of the final deposits of terminalcontact material to the PLM. As shown in FIG. 4, following the openingof this window 26 in the photoresist a 125 to 150 micron thick lead-tindeposit 27 is made over the entire surface of the photoresist layer 25by any suitable deposition technique, such as for example, evaporation.This deposit 27 also forms on the surface of the PLM 24 exposed throughthe window 26. The lead-tin deposit in the region of the window 26 isless then the thickness of the photoresist layer 25 and is thus shown asthe depressed layer 27a. Following this lead-tin evaporation and depositof layers 27 the coated semiconductor wafer 20 is heated in any suitablemanner, which can be for example a suitable oven or other heated chamber(not shown), to a temperature of approximately 360° C. which is abovethe melting temperature of the lead-tin deposit, but below thedegradation temperature, i.e. that the temperature below which adverseaffects such as oxidation, depolymerization or other significant changesin the material characteristics occur of the polyimide 21 or thephotoresist layer 24. It should be noted that such heating of the devicemust be above the melting point of the metal but below that temperaturewhich will adversely affect the semiconductor circuit 23 or any of thepreviously deposited materials. Preferably this melting is, in thisinstance, achieved by heating the coated device in a furnace containinga hydrogen atmosphere at 360°. Because of the confining walls of thephotoresist 25 around the opening 26 the lead-tin deposit upon meltingforms a terminal solder ball 27b in the opening 26 which upon cooling isfixedly adhered to the underlying PLM layer 24. It should be noted thatsolder ball 27b has considerably more mass than the deposited layer 27a.This is due to the flowing of the melted material and surface tension inthe melted material which causes the ball 27b to pull additionalmaterial from the surface of the photoresist. That material which is notattracted into the opening 26 by virtue of surface tension of the ball27 is caused by the same surface tension to be gathered into a pluralityof small globules 29 as shown in FIG. 5.

It should be noted that such heating can be localized on a particularportion of the deposited lead-tin layer by, for example uses a laser ora micro-flame apparatus. Again, the temperature of any of the underlyinglayers must not be caused to reach the degradation temperature. Stillfurther it should be noted that materials other than lead-tincombinations can be used. The sole requirement being that the meltingtemperature of the selected material being below that of either thepolymerization or degradation temperature of the underlying layers.

Since surface tension causes the lead-tin in the opening 25 to bebuilt-up into heights well above the photoresist thickness the need forthick initial accumulations is eliminated and the initial deposit needonly be about one-half that of required by prior art techniques. Becausethe lead-tin deposit on the surface of layer 25, due to surface tension,forms globules 29 on the surface of the photoresist layer 25, extensiveportions of the surface of layer 25 are exposed making it more availablefor rapid chemical stripping. Since the photoresist layer 25 has notbeen altered by the heating action it can be readily stripped, using astandard photoresist stripper within a period of between 1 and 4minutes. Such stripping of the photoresist layer also removes theglobules of solder 29 accumulated on the surface while leaving untouchedthe solder ball 27b, as shown in FIG. 6. After the removal of thephotoresist layer 25 the PLM layer 24 of chrome-copper-gold is exposedeverywhere except under ball 27b, as shown in FIG. 6. This undesired andnow exposed portion of PLM layer 24 can be removed from the surface ofthe polyimide layer 21 by a standard etch procedure using eitherpotassium cyanide, gold, ammonium hydrosulfate or ammonium hydroxideplus water for the copper or potassium permanganate for the chrome.

This results in a single solder ball mounted on a remaining portion 24aof the PLM layer 24 which extends through the opening 22 in thepolyimide layer 21. The solder ball is now suitable for connection to anexternal circuit, such as a printed circuit board by conventional means.

Tests performed on such devices using the above described process havefound that by slightly tilting the semiconductor wafer at an angle, ofabout 20° or more, from the horizontal during heating will cause theexcess melted solder, to run off the surface of the device leaving onlythe material contained within the opening 26 in the photoresist. Thus,globules 29 run off the surface of the photoresist. Such globules canalso be forced off the surface by using air jets or other suchtechniques.

Turning now to FIGS. 8 through 13, there is described the embodimentpreferred by the inventors and which is a variation in the processdescribed in FIGS. 1 through 7. FIG. 8 shows a silicon semiconductorwafer 40 having a polyimide layer 41 deposited thereon in which anopening 42 has been previously opened through photolithographictechniques. Following this as shown in FIG. 9 a suitable image definingmask 43 such as photoresist or molybdenum is disposed as previouslydescribed over the polyimide 21 so that all of the layer 41 except for adefined region around opening 42 is covered. A pad limiting metallurgyor PLM deposit 44 is now laid down over and around the opening 42 usingstandard deposition techniques. Subsequently, the mask 43 is removedusing standard etching procedures or other removal techniques suitablefor the mask used, and as shown in FIG. 10, using standard techniques, alayer of photoresist 45 is laid down on the surface of layer 41. Thislayer 45 is provided with an opening 46 which is larger than that of thepad limiting metallurgy deposit 44 placed in and around the opening 22and exposes a ring 47 of the polyimide surface around the PLM deposit44. This means that photoresist masks need be less restrictive as tomatching up to the dimensional aspects of the pad limiting metallurgy.Thus, looser groundrules, i. e. dimensional differences, are permittedto be used with this process as compared to the process described inconjunction with FIGS. 1 to 7 and which taught that the photoresistcovered a portion of the pad limiting metallurgy.

Following the deposit of the photoresist layer 45 a layer 48 of lead-tinis deposited, by any convenient technique, over the photoresist 45 andthe opening 46 as shown in FIG. 11. Following the deposit of thelead-tin layer 48 the coated wafer 40 is heated to melt the layer 48.This melting of layer 48 is accomplished in the manner described inconjunction with FIGS. 1 to 7 and forms a ball of solder 48b over thePLM 44. Due to the surface tension of the melted lead-tin layer theportion 48a, deposited in opening 46, contracts from the exposed surfaceof the polyimide ring 47 around the PLM 44 and collates into ahemispherical ball 48b positioned exactly over and on the PLM 44. Thusno excess lead-tin remains on the exposed polyimide surface 47 withinthe confines of the photoresist defined opening 46. The remainingdeposit of lead-tin on the surface of the photoresist layer 45 is formedby this heating step into globules 49.

Following this heating and balling of the deposited lead-tin layer 48the photoresist material 45 can be removed by immersing the unit into astandard photoresist removal bath. At that point the device is completedand no further processing is required.

It should be understood that, at times, it may not be necessary toremove the masking material after the heating of the unit especiallywhen the unit is heated in such a way that all the excess metal iscaused to run off the surface of the masking material during the heatingstep.

Thus, there has been described two different processes utilizing imagedefining masks which are not significantly affected at the temperaturesrequired to melt the lead-tin deposit. Photoresist materials suitablefor such a purpose include dry film, such as Dupont Vacrel, Dynachem orliquids such as Ciba-Geigy #348. All of these are particularly usefulbeing suitable for image creation of photolithographic images therein.

The processes described herein permits reduction in the evaporation timeby permitting thinner lead-tin alloy deposits and provides forsignificant surface clearance of the photoresist layer in a brief periodwhile retaining the required terminal pads, thus permitting thestripping of the photoresist to occur in a significantly shorter periodof time, i.e., 1 to 4 min., versus approximately 30 min. or more usingthe prior art techniques.

It should be noted that the resist utilized can be water soluble,halogenated or other types of solvents can be used and which were notusable by the prior art processes.

It should be especially noted that this process can be adapted toproduced lines as well as individual pads and that while the inventionherein is described with references to the preferred embodiments of theinvention it should be understood that numerous variations may be madein these processes without departing from the scope of the invention asdefined in the appended claims.

I claim:
 1. A lift-off process for removing a metallic layer depositedover a layer of masking material deposited on a semiconductor devicecomprising the steps of:depositing a layer of masking material on thesurface of the semiconductor device, creating an opening in the layer ofmasking material, and depositing a layer of metal over said layer ofmasking material and in said opening, heating said metal layer to causemelting of said metal layer in said opening, removing said metal outsideof said opening.
 2. The process of claim 1 wherein the masking materialis a photoresist.
 3. The process of claim 2 wherein the opening in themasking material is created by a photolithographic process.
 4. Theprocess of claim 2 wherein said layer of metal has a melting point belowthe degradation temperature of the photoresist.
 5. The process of claim4 wherein said heating is achieved by using a heated chamber.
 6. Theprocess of claim 4 wherein said heating is achieved by using a laserbeam.
 7. A lift-off process for removing a metallic layer deposited overa layer of masking material deposited on a semiconductor devicecomprising the steps of:creating a layer of insulating material on aselected surface of the device, forming openings in selected regions ofsaid insulating layer, depositing a first metallic layer in saidopening, depositing a layer of masking material on the surface of thelayer of insulating material, creating a plurality of openings in thelayer of masking material, and depositing a second metallic layer oversaid layer of masking material and in said openings, characterized byheating said second metallic layer to cause melting of said secondmetallic layer, and removing said layer of masking material.
 8. Theprocess of claim 7 wherein said insulating layer and said layer ofmasking material are different polymers.
 9. The process of claim 8wherein said openings in said insulating layer and said layer of maskingmaterial are formed photolithographically.
 10. The process of claim 9wherein said first metallic layer is comprised of chrome-copper-gold orchrome-copper-chrome.
 11. The process of claim 10 wherein said secondmetal layer is comprised of lead and tin.
 12. The process of claim 10wherein said second metallic layer has a melting point below the meltingpoint of the first metallic layer and below the degradation temperatureof the polymers.
 13. The process of claim 12 wherein said layer ofmasking material is removed from the layer of insulating material by achemical stripping action.
 14. The process of claim 10 wherein saidsemiconductor device is positioned at an angle of more than twentydegrees to the horizontal while said second metallic layer is beingheated.
 15. The process of claim 14 wherein said device is heated in achamber to a temperature of between 300° C. and 450° C.
 16. The processof claim 10 wherein said second metallic layer is heated with a laser.17. The process of claim 7 wherein said insulating layer is composed ofa glass.
 18. The process of claim 7 wherein said second metallic layeris approximately one-half the thickness of said layer of maskingmaterial.
 19. A lift-off process for removing a metallic layer depositedover a photoresist layer deposited on a semiconductor device comprisingthe steps of:creating a layer of insulating material on a surface of thedevice, forming openings in selected regions of said insulating layer,depositing a first metallic layer in said openings, depositing a layerof photoresist, having a polymerization temperature and adepolymerization temperature, on the surface of said insulating layer,photolithographically creating a plurality of openings in thephotoresist layer, over selected regions of said first depositedmetallic layer, depositing a second metallic layer over said photoresistlayer and in said openings in said photoresist layer, said second layerhaving a melting point less than the melting point of said firstmetallic layer and less than the polymerization temperature and thedepolymerization temperature of the photoresist, heating said secondmetal layer to cause melting of said second metal layer, and removingsaid photoresist from the surface of said insulating layer by using achemical stripping agent.